Apparatus for evaluating metalized layers on semiconductors

ABSTRACT

An apparatus for characterizing multilayer samples is disclosed. An intensity modulated pump beam is focused onto the sample surface to periodically excite the sample. A probe beam is focused onto the sample surface within the periodically excited area. The power of the reflected probe beam is measured by a photodetector. The output of the photodetector is filtered and processed to derive the modulated optical reflectivity of the sample. Measurements are taken at a plurality of pump beam modulation frequencies. In addition, measurements are taken as the lateral separation between the pump and probe beam spots on the sample surface is varied. The measurements at multiple modulation frequencies and at different lateral beam spot spacings are used to help characterize complex multilayer samples. In the preferred embodiment, a spectrometer is also included to provide additional data for characterizing the sample.

TECHNICAL FIELD

[0001] The subject invention relates to a method and apparatusparticularly suited for the nondestructive characterization of opaquelayer structures on semiconductor samples.

BACKGROUND OF THE INVENTION

[0002] There is a great need in the semiconductor industry for metrologyequipment which can provide high resolution, nondestructive evaluationof product wafers as they pass through various fabrication stages. Inrecent years, a number of products have been developed for thenondestructive evaluation of semiconductor samples. One such product hasbeen successfully marketed by the assignee herein under the trademarkTherma-Probe. This device incorporates technology described in thefollowing U.S. Pat. 4,634,290; 4,646,088; 5,854,710 and 5,074,669. Thelatter patents are incorporated herein by reference,

[0003] In the basic device described in the patents, an intensitymodulated pump laser beam is focused on the sample surface forperiodically exciting the sample. In the case of a semiconductor,thermal and plasma waves are generated in the sample which spread outfrom the pump beam spot. These waves reflect and scatter off variousfeatures and interact with various regions within the sample in a waywhich alters the flow of heat and/or plasma from the pump beam spot.

[0004] The presence of the thermal and plasma waves has a direct effecton the reflectivity at the surface of the sample. Features and regionsbelow the sample surface which alter the passage of the thermal andplasma waves will therefore alter the optical reflective patterns at thesurface of the sample. By monitoring the changes in reflectivity of thesample at the surface, information about characteristics below thesurface can be investigated.

[0005] In the basic device, a second laser is provided for generating aprobe beam of radiation. This probe beam is focused colinearly with thepump beam and reflects off the sample. A photodetector is provided formonitoring the power of reflected probe beam. The photodetectorgenerates an output signal which is proportional to the reflected powerof the probe beam and is therefore indicative of the varying opticalreflectivity of the sample surface.

[0006] The output signal from the photodetector is filtered to isolatethe changes which are synchronous with the pump beam modulationfrequency. In the preferred embodiment, a lock-in detector is used tomonitor the magnitude and phase of the periodic reflectivity signal.This output signal is conventionally referred to as the modulatedoptical reflectivity (MOR) of the sample.

[0007] This system has the advantage that it is a non-contact,nondestructive technique which can be used on product wafers duringprocessing. Using lasers for the pump and probe beams allows for verytight focusing, in the micron range, to permit measurements with highspatial resolution, a critical requirement for semiconductor inspection.The prior system has been used extensively in the past to monitor levelsof ion doping in samples since the modulated optical reflectivity isdependent on ion dopant levels in the sample. This dependence isrelatively linear for the low to mid-dose regimes (10¹¹ to 10¹⁴ions/cm²). At higher dopant concentrations, the MOR signal tends tobecome non-monotonic and further information is needed to fully analyzethe sample.

[0008] One approach for dealing with the problem of monitoring sampleswith high dopant concentrations is to measure the DC reflectivity ofboth the pump and probe beams in addition to the modulated opticalreflectivity signal carried on the probe beam. Using the DC reflectivitydata at two wavelengths, some ambiguities in the measurement can oftenbe resolved. The details of this approach are described in U.S. Pat. No.5,074,669, cited above.

[0009] Semiconductor fabrication technology is increasing in complexityat a rapid pace. Various multilayer structures are being developed whichmakes testing more difficult. In addition, manufacturers are seeking toincrease yields by fabricating chips on larger diameter wafers. As thediameter of the semiconductor wafers increases, the cost and value ofeach wafer increases. When using large, valuable and expensive wafers,it is no longer economically viable for manufacturers to rely on anyforms of destructive testing methodologies. Therefore, there is a greatneed to provide equipment which can characterize complex structures withmany unknowns or variables in a nondestructive manner.

[0010] Inspection problems also arise where metalized layers aredeposited on semiconductors. If a typical metal layers is more than 100angstroms thick, it will generally be opaque to more commonly usedoptical wavelengths. Therefore, equipment designed to monitor relativelytransparent oxide layers cannot be effectively used to inspect metalizedlayers. Therefore, some new methodologies are required in order toinspect semiconductors with metalized layers. These layers can be formedfrom materials, such as aluminum, titanium, titanium nitride (TiN) andtungsten silicide (WSi).

SUMMARY OF THE INVENTION

[0011] In order to obtain sufficient information to characterize morecomplex samples, a system has been developed which substantiallyincreases the amount of data that can be collected. The system of thesubject invention includes an intensity modulated pump laser beam whichis focused onto the sample in a manner to periodically excite thesample. A probe laser beam is focused onto the sample within theperiodically heated area. A photodetector is provided for monitoring thereflected power of the probe beam and generating an output signalresponsive thereto. The output signal is filtered and processed toprovide a measure of the modulated optical reflectivity of the sample.

[0012] In accordance with the subject invention, the device furtherincludes a steering means for adjusting the relative position of pumpand probe beam spots on the sample surface. In the preferred embodiment,the steering means is used to move the beam spots from an overlapping,aligned position, to a separation of up to about 10 microns.Measurements can be taken as the separation of the beam spots isgradually changed or at discrete separation intervals.

[0013] This approach is particularly useful for monitoring thedeposition of opaque, thin metal films. More specifically, themeasurements taken at different spatial distances between the pump andprobe beam spots can be used to help more accurately characterize thethermal diffusivity of the layer. This information can then be used bythe processor to more accurately characterize the sample.

[0014] It should be noted that the concept of taking a measurement witha probe beam displaced from a pump can be found in the prior art. Forexample, in U.S. Pat. Nos. 4,521,118 and 4,522,510, both assigned toassignee herein and incorporated by reference, deformations at thesample surface, induced by periodic heating, are measured using a probebeam displaced from the pump beam. In the latter patents, periodicangular deviations of the probe beam are monitored. However, the latterpatents do not teach or suggest that it would be desirable to takemultiple measurements as the displacement between the two beams spotsare varied.

[0015] Obtaining measurements from a probe beam displaced from a pumpbeam is also disclosed in U.S. Pat. No. 5,228,776, assigned to assigneeherein and incorporated by reference. In this patent, an effort is madeto align the pump and probe beams at the opposed ends of elongatedconductive features within the sample. Further, the focal planes of thetwo beams are displaced vertically. In the principal embodiment of theU.S. Pat. No. 5,228,776 patent, the lateral spacing between the beams isselected and then fixed. There is no teaching in the U.S. Pat. No.5,228,776 patent that it would be desirable to take multiplemeasurements as the displacement between the two beams is varied.

[0016] In the preferred embodiment of the subject invention, furtherinformation can be obtained by varying the modulation frequency of thepump beam. While it has been known that obtaining information as afunction of modulation frequency is useful, the subject inventionexpands upon the past teachings by increasing the modulation range. Inparticular, in the prior art, the modulation range was typically in the100 KHz to 1 MHz range. Some experiments utilized modulation frequencyas high as 10 MHz. In the subject device, it has been found useful totake measurements with modulation frequencies up to 100 MHz range. Atthese high frequencies, the thermal wavelengths are very short, enablinginformation to be obtained for thin metal layers on a sample, on theorder of 100 angstroms.

[0017] In the preferred embodiment of the subject invention, furtherinformation can be obtained by varying the spot sizes of either the pumpor probe beams. Varying the spot size of the pump beam will vary thepropagation characteristics of the thermal waves. Varying the spot sizeof the probe beam will vary the sensitivity of the system with respectto the depth of detection. By taking measurements at different spotsizes, some depth profiling information can be recorded and used tocharacterize the sample.

[0018] In the preferred embodiment of the subject invention, stillfurther information can be derived by obtaining independent reflectivitymeasurements at a plurality of wavelengths. More specifically, thesubject apparatus can further include a polychrornatic light sourcegenerating a second probe beam which is directed to the sample surface.The reflected beam is captured by a detector which is capable ofmeasuring power as a function of wavelength. These added measurementscan also be used to help better resolve ambiguities in the analysis andimprove the characterization of the sample.

[0019] It is also possible to add additional measurement modules whichmeasure either reflectivity or ellipsometric parameters as a function ofangle of incidence. Further, the system can also be used. to monitor theperiodic angular deviations of the probe beam to derive additionalinformation.

[0020] Further objects of the subject invention will become apparentfrom the following detailed description, taken in conjunction with thedrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic block diagram of the apparatus of thesubject invention.

[0022]FIG. 2 is a schematic diagram of a frequency synthesizer andlock-in amplifier for electronically down mixing the high frequency MORsignal to create a low frequency heterodyne signal output.

[0023]FIG. 3 is a schematic diagram of a quad cell photodetector.

[0024]FIG. 4 is a schematic diagram of an array spectrometer.

[0025]FIG. 5 is a schematic diagram .of the detection module forsimultaneously determining the reflectivity of rays within a beamstriking the sample at multiple angles of incidence.

[0026]FIG. 6 is a schematic diagram of a detection module fordetermining ellipsometric parameters of a sample by analyzing areflected probe beam and which includes integrated data over multipleangles of incidence with respect to the sample surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027] Turning to FIG. 1 there is shown a schematic diagram of theapparatus 10 of the subject invention. The apparatus 10 is intended topermit testing of a semiconductor: sample 12 and in particular can beused to derive information about layers on the sample. Sample 12 isshown resting on a controllable stage 14. Stage 14 is capable of precisemovements and functions to scan the sample with respect to the laserbeams.

[0028] In the preferred embodiment, the apparatus 10 includes a pumplaser 20 for exciting the sample and a probe laser 22 for monitoring thesample. Gas, solid state or semiconductor lasers can be used. Asdescribed in the assignees earlier patents, other means for exciting thesample can include different sources of electromagnetic radiation orparticle beams such as from an electron gun.

[0029] In the preferred embodiment, semiconductor lasers are selectedfor both the pump and probe lasers due to their reliability and longlife. In the illustrated embodiment, pump laser 20 generates a nearinfrared output beam 24 at 790 nm while probe laser 22 generates avisible output beam 26 at 670 nm. Suitable semiconductor lasers for thisapplication include the Mitsubishi ML6414R (operated at 30 mW output)for the pump laser and a Toshiba Model 9211 (5 mW output) for the probelaser. The outputs of the two lasers are linearly polarized. The beamsare combined with a dichroic mirror 28. It is also possible to use twolasers with similar wavelengths and rely on polarization discriminationfor beam combining and splitting.

[0030] Pump laser 20 is connected to a power supply 30 which is underthe control of a processor 32. The output beam of laser 20 is intensitymodulated through the output of power supply 30. The modulationfrequency has a range running from 100 KHz to 100 MHz. In the preferredembodiment, the modulation frequency can be set up to 125 MHz. Asdescribed in the above cited patents, if an ion laser is used togenerate the pump beam, the intensity modulation can be achieved by aseparate acousto-optic modulator.

[0031] Prior to reaching the beam combining mirror 26, the probe beam 24passes through a tracker 40 and a shutter 42. Tracker 40 is used tocontrol the lateral position of beam 24 with respect to the probe beamas discussed more fully hereinbelow. The shutter 42 is used to block thepump beam when other measurements which do not require periodicexcitation are being taken.

[0032] The beam 26 from probe laser 22 is turned by mirror 46 and passedthrough a collimator 48 which is used to match the focal plane of theprobe spot with the pump laser spot. A shutter 50 is also located in thepath of the probe beam 26 to block the probe beam when measurements thatdo not use the probe are being taken, so that no stray probe beam lighthits the detector.

[0033] After the beams 24 and 26 are combined, they are redirectedthrough polarizing beam splitter 54 to another beam splitter 56. Inbetween, the beams pass through a quarter-wave plate 58 for rotating thepolarization of the beams by 45 degrees. The beams are directed down tothe sample 12 through a microscope objective 60. Objective 60 has a highn.a., on the order of 0.9, and is capable of focusing the beam to a spotsize on the order of a few microns and preferably close to one micron indiameter. The spacing between the objective and the sample is controlledby an autofocus system described in greater detail hereinbelow.

[0034] The returning reflected beams 24 and 26 pass through thequarter-wave plate 58 a second time, resulting in another 45 degreepolarization rotation. This second rotation allows the beams to passthrough the beam splitter 54. A filter 64 is provided to remove the pumpbeam light 24 allowing the probe beam light to fall on the photodetector70. Detector 70 provides an output signal which is proportional to thepower of the reflected probe beam 26. Detector 70 is arranged to beunderfilled so that its output can be insensitive to any changes in beamdiameter or position. In the preferred embodiment, detector 70 is a quadcell generating four separate outputs. When used to measure reflectedbeam power, the output of all four quadrants are summed. When thesubject apparatus is operated to measure beam deflection, the output ofone adjacent pair of quadrants is summed and subtracted from the sum ofthe remaining pair of quadrants. This later measurement will bediscussed below.

[0035] The output of the photodetector 70 is passed through a low passfilter 71 before reaching processor 32. One function of filter 71 is topass a signal to the processor 32 proportional to the DC power of thereflected probe. A portion of filter 71 also functions to isolate thechanges in power of the reflected probe beam which are synchronous withthe pump beam modulation frequency. In the preferred embodiment, thefilter 71 includes a lock-in detector 72 for monitoring the magnitudeand phase of the periodic reflectivity signal. Because the modulationfrequency of pump laser can be so high, it is preferable to provide aninitial down-mixing stage for reducing the frequency of detection.

[0036] A schematic diagram of the frequency generation and detectionstage is illustrated in FIG. 2. As shown therein, a frequencysynthesizer 73 is provided for generating the various pump beammodulation frequencies. Synthesizer 73 is under the control of processor32 and can generate an output from 100 KHz to at least 125 KHz. Thisoutput is delivered as a signal to the power supply 30 of laser 20.

[0037] Synthesizer 73 also generates an electronic heterodyne signal fordelivery to the lock-in amplifier 72. The heterodyne signal will beclose to, but different from the signal sent to the pump laser. Forexample, the heterodyne signal can be 10 KHz higher than the signal sentto the pump laser.

[0038] The heterodyne signal from the synthesizer is combined with theoutput from the signal detector 70 in a mixer 74. The output of themixer will include signal components at both the sum and difference ofthe two input signals. The difference signal will be at the relativelylow frequency of 10 KHz. All the signals are passed through a low passfilter 75 to eliminate the high frequency components from thesynthesizer and the detector. The low frequency signal is thendemodulated by demodulator 76. The outputs of demodulator 76 are the“in-phase” and “quadrature” signals typical of a lock-in amplifier. Thein-phase and quadrature signals can be used by processor 32 to calculatethe magnitude and the phase of the modulated optical reflectivitysignal.

[0039] In initial experiments, a model SR844 lock-in detector fromStanford Research Systems was utilized. This device utilizes acombination of analog and digital techniques to permit operation over awide frequency range. In this device, an internal frequency synthesizeris used to modify the incoming reference signal from the synthesizer togenerate two reference signals which differ in phase by 90 degrees.These reference signals are mixed with the incoming signals from thedetector 70. After filtering, the low frequency signals are digitized bytwo 16-bit analog-to-digital converters. The digital low frequencysignals are supplied to a DSP chip for analysis.

[0040] As an alternative to using an electronic heterodyne down-mixingsystem, it is also possible to reduce the frequency of detection usingan optical heterodyne approach. Such an optical approach is disclosed inU.S. Pat. No. 5,408,327, incorporated herein by reference. In the lattersystem, both of the laser beams are modulated but at slightly differentfrequencies. Both beams generate thermal and plasma waves at theirrespective modulation frequencies. The beam from one laser picks up anintensity modulation upon reflection due to the modulated opticalreflectivity induced in the sample by the other beam. The MOR signalpicked up upon reflection “mixes” with the inherent modulation of thebeam, creating additional modulations in the beam at both the sum anddifference frequency. This process is analogous to electricalheterodyning. The difference or “beat” frequency is much lower thaneither of the initial beam modulation frequencies and can therefore bedetected by a low frequency lock-in amplifier.

[0041] It should be noted that in the latter arrangement, both beamscarry the desired MOR signal and either one or both of the reflectedbeams can be measured. Therefore, in the latter system, the term probebeam could be applied to either laser beam. It should also be noted thatwhen some samples are exited with two modulation frequencies, nonlineareffects in the sample may result in periodic excitation at the sum anddifference frequencies in the sample. This effect is usually relativelysmall.

[0042] To insure proper repeatability of the measurements, the signalsmust be normalized in the processor. As noted above, the DC reflectivityof the probe beam is derived from detector 70. In addition, the DCoutput powers of the pump and probe lasers are monitored by incidentpower detectors 84 and 86 respectively. A wedge 88 functions to pick offabout one percent of the incident beam power and redirects it to an edgefilter 90 for separating the two beams. The outputs of the detector 84and 86 are passed through the low pass portion of filter 71 and into theprocessor 32.

[0043] The signals are further normalized by taking a measurement of thepower of the pump beam 24 after it has been reflected. This measurementis used to determine the amount of pump energy which has been absorbedin the sample. The pump beam reflected power is measured by detector 94.Wedge 88 functions to pick off about one percent of the returning beamswhich are redirected to edge filter 96. The DC signal for both theincident pump and probe beam powers as well as the reflected beam powersare used to correct for laser intensity fluctuations and absorption andreflection variations in the samples. In addition, the signals can beused to help calculate sample parameters.

[0044] An autofocus mechanism is used to maintain the spacing betweenthe objective 60 and the sample 12 to be equal to the focal length ofthe objective. This distance can be maintained to less than onehundredth of a micron.

[0045] The autofocus mechanism includes a servo motor 108 for varyingthe vertical position of the objective 60. The servo is driven by ananalog detection loop which determines if the objective 60 is properlyfocusing the probe beam. As seen in FIG. 1, a small portion of thereflected probe beam light picked off by the wedge 88 is redirected byfilter 96 into the main elements of the autofocus detection loop. Theprobe beam is focused by a lens 98 through a chopper wheel 104 locatedin the focal plane of the lens 98. The light passing the chopper wheel104 is imaged on a split-cell photodetector 106.

[0046] If the objective 60 is out of focus, there will be a phasedifference in the light striking the two sides of the split celldetector 106 which is measured by a phase detector 109. The phasedifference is used as an input to an amplifier 110 which in turn drivesthe servo 108. This approach to autofocusing is known as the automatedFoucault knife edge focus test. This system can also be used tointentionally defocus the beams, thereby varying the spot sizes of thepump and probe beams. This adjustment can be used to gain furtherinformation as discussed below.

[0047] In the preferred embodiment, a polychromatic, or white lightsource 120 is provided to illuminate the sample to permit viewing on aTV monitor (not shown). The beam 122 from the white light source 120 ispassed through a shutter 124. When measurements with the lasers 20 and22 are being taken, the shutter is closed so no stray white light getsinto the detection system.

[0048] A portion of the white light returning from the sample isredirected by a beam splitter 130 to a CCD camera 132 for generating animage for the TV monitor. The beam will pass through a CCD lens 134 anda filter 136. Filter 136 functions to reduce the amount of laser lightreaching the camera. In the preferred embodiment, a portion of the lightis redirected by a beam splitter 140 into an array spectrograph 142 morefully described herein below.

[0049] Having described the basic elements of the subject invention, itsoperation will now be discussed.

[0050] In use, a sample 12 is placed on the stage 14. Measurements atvarious points on the sample can be made by rastering the stage to thedesired location. During initial set up of a fabrication procedure, arelatively high number of test points may be desired. Once thefabrication procedure is up and running, only spot checks of the waferare typically made. This will often entail measuring from five totwenty-five spots on a wafer.

[0051] At each measurement point, the pump and probe laser beams 24 and26 are activated. In a typical measurement, the beams will be initiallycolinearly aligned. Exact overlap will typically provide the strongestmodulated optical reflectivity signal. Measurements can then be taken ata plurality of modulation frequencies of the pump beam. The range ofmodulation frequencies and the number of frequencies is selected by theoperator. As noted above, in the preferred embodiment, the range of pumpbeam modulation frequencies covers from 100 KHz to 125 MHz. The outputfrom the photodetector 70 is passed through the filter 71 (includinglock-in 72) to the processor which records the magnitude and/or phase ofthe modulated optical reflectivity signal for each of the modulationfrequencies selected.

[0052] In accordance with the subject invention, once the desiredmeasurements are completed with the beam spots in an overlapping,aligned position, the processor signals the tracker 40 to adjust theposition of the pump beam 24 so that pump and probe beam spots on thesample surface are laterally displaced. In one possible scenario, thebeams are initially displaced a distance of one micron. Once in thisposition, a series of measurements are taken at different modulationfrequencies in the manner described above. Once again, the processorwill record the modulated optical reflectivity signal at each of thesemodulation frequencies.

[0053] In accordance with the subject invention, once the second set ofmeasurements are complete, the processor will again command the tracker40 to further separate the pump and probe beam spots to a distance oftwo microns. Measurements will then be taken at this two micron spacingand at subsequent spacings, each time increasing the spacing by onemicron. It is envisioned that for complex samples, measurement might betaken at successive increments of 0.5 microns. Alternatively,measurements can be taken as the separation of the two beam spots iscontinuously increased. The span of spacings between beam spots canrange from overlapping to about 10 microns.

[0054] It is envisioned that the user will be able to determine whatsort of scanning algorithm is best suited to the particular testsituation. Variables such as separation distance and number ofmeasurements at each separation can be entered by the user through asoftware interface.

[0055] Once all of the measurements at various spacings and modulationfrequencies have been taken and stored, the processor will attempt tocharacterize the sample. Various types of modeling algorithms can beused depending on the complexity of the sample. Optimization routineswhich use iterative processes such as least square fitting routines aretypically employed. One example of this type of optimization routineused for thermal wave analysis is described in “Thermal WaveMeasurements and Monitoring of TASi_(X) Film Properties,” Smith, et.al., Journal of Vacuum Science and Technology B. Vol. 2, No. 4, p. 710,1984. An optimization routine used for more general optical analysis canbe found in “Multiparameter Measurements of Thin Films UsingBeam-Profile Reflectivity,” Fanton, et. al., Journal of Applied Physics,Vol. 73, No. 11, p.7035, 1993.

[0056] It has been found that measurements taken at different beam spotspacings are particularly useful in characterizing thermal diffusivityof a sample. Identification of this parameter can be used to determinethe structure of the sample. In addition, measurements at very highmodulation frequencies are useful for very thin films, since thewavelength of the thermal and plasma waves is very short.

[0057] In the preferred embodiment of the subject invention, furtherinformation can be obtained by varying the spot sizes of either of bothof the pump and probe beams. Varying the spot sizes of the beams willvary the propagation and detection characteristics of the thermal waves.In particular, when using a highly focused pump beam spot size, thethermal waves will tend to propagate in all directions (3 dimensionally)away from the area of focus. In contrast, as the pump beam spot sizeincreases, the thermal waves will tend to propagate in a more1-dimensional manner, perpendicular to the area of focus.

[0058] With respect to the detection, a very large probe beam spot tendsto see mostly the 1-dimensional component of the thermal wave. Incontrast, a highly focused probe beam, being more localized, issensitive to the 3-dimensional character of the thermal waves. Thisvariation in the generation and detection characteristics of thermalwaves is characterized by the square root of the sum of the squares ofthe pump and probe beam diameters. When this value is small compared tothe thermal diffusion length, the measurement is 3-dimensional incharacter and when the value is large compared to the thermal diffusionlength, the measurement is 1-dimensional in character. By takingmeasurements at different pump and/or probe beam spot sizes, some depthprofiling information can be recorded and used to characterize thesample.

[0059] As noted above, the spot size of both the beams can be controlledby the autofocus system. In order to increase the size of the beamspots, the processor can add an offset to the focusing algorithm whichwould defocus the beams. The beam spots can be made of different sizesby adjusting the collimator 48. In the preferred embodiment,measurements are taken and recorded at various beam spot sizes rangingfrom one micron to ten microns. This additional information can be usedto characterize the sample.

[0060] In the preferred embodiment of the subject invention, stillfurther measurements can be taken to reduce the ambiguities of analysis.More specifically, in addition to the modulated optical reflectivitymeasurements, it is also desirable to monitor the periodic angulardeflections of the probe beam due to deformations in the surface of thesample induced by the periodic heating. This type of measurement isdescribed in detail in U.S. Pat. Nos. 4,521,118 and 4,522,510, citedabove. As described in those patents, because of the thermal expansionproperties of samples, the periodic heating by the pump beam will createa time varying “bump” in the sample surface. If the pump and probe beamare spaced apart, the probe beam will undergo periodic angulardeviations at the frequency of the modulated heating. These angulardeviations can be measured by a split cell detector. The output of thesplit cell is sent to the filter and the processor. The processorfunctions to calculate the magnitude and phase of the deflection signalthat is synchronous with the pump beam modulation frequency.

[0061] As noted earlier, detector 70 is preferably a quad cell detector,or a detector with four quadrants each generating separate outputsignals. Deflections of the probe beam in both the X and Y axis due toangular deviations of the probe beam can be measured with such adetector. More specifically and as shown in FIG. 3, to determine thedeflection of the probe beam in the X axis, the outputs of quadrants 200and 202 are summed and subtracted from the sum of quadrants 204 and 206.Alternatively, to determine the deflection of the probe beam in the Yaxis, the outputs of quadrants 200 and 204 are summed and subtractedfrom the sum of quadrants 202 and 206. It should be noted that thesetype of measurements function to cancel out any changes in the reflectedpower of the beam. As noted above, in operation, the output of all fourdetector quadrants are fed to the filter 71 and processor 32. If theoutput of the four quadrants is summed, the MOR signal is obtained. Ifthe quadrant halves are summed and subtracted, the probe beam deflectionsignal can be obtained.

[0062] In use, the deflection signal can be recorded for each of thebeam spot positions and modulation frequencies. In practice, there willusually be little or no deflection signal when the two beam spots arecolinearly aligned. However, as soon as the centers of the beam spotsare separated, a signal can be often be detected, even if the beams arein a partially overlapping configuration.

[0063] In the preferred embodiment, the subject apparatus furtherincludes a spectrometer for providing additional data. As noted above, awhite light source 120 is necessary for illuminating the sample fortracking on a TV monitor. This same light source can be used to providespectral reflectivity data.

[0064] As seen in FIG. 1, a beam splitter can be used to redirect aportion of the reflected white light to a spectrometer 142. Thespectrometer can be of any type commonly known and used in the priorart. FIG. 4 illustrates one form of a spectrometer. As seen therein, thewhite light beam 122 strikes a curved grating 242 which functions toangularly spread the beam as a function of wavelength. A photodetector244 is provided for measuring the beam. Detector 244 is typically aphotodiode array with different wavelengths or colors falling on eachelement 246 in the array. The outputs of the diode array are sent to theprocessor for determining the reflectivity of the sample as a functionof wavelength. This information can be used by the processor during themodeling steps to help further characterize the sample.

[0065] It is also possible to provide a mechanism for measuring samplereflectivity as a function of the angle of incidence of the beam. Toachieved this goal, a portion of the reflected probe beam light 26 canbe picked off by wedge 300. This light is sent to a beam profilereflectometer module 302 as more clearly shown in FIG. 5. When takingthese measurements, the pump beam is, preferably turned off so that theprobe beam will not be modulated.

[0066] Module 302 is of the type described in U.S. Pat. No. 4,999,014assigned to the assignee herein and incorporated. by reference. Asdescribed therein, if a probe beam is focused onto a sample with astrong lens, various rays within the beam will strike the sample surfaceat a range of angles of incidence. If the beam is properly imaged with arelay lens 306, the various rays can be mapped onto a linear photodiodearray 308. The higher angles of incidence rays will fall closer to theopposed ends of the array. The output from each element 309 in the diodearray will correspond to different angles of incidence. Preferably, twoorthoganlly disposed arrays 308 a and 308 b are provided to generateangle of incidence information in two axes. A beam splitter 310 is usedto separate the probe beam into two parts so both axes can be detectedsimultaneously. The output of the arrays will supplied to the processor32 for storage. The data can be used to further characterize the sample.

[0067] In still a further preferred embodiment, ellipsometricinformation can be derived from the sample using the reflected probebeam. As in the previous example, the pump beam should be turned off forthese measurements. As seen in FIG. 1, a portion of the reflected probebeam can be picked off by a beam splitter 400 and redirected to a beamprofile ellipsometer 402 as shown in FIG. 6. A suitable beam profileellipsometer is described in U.S. Pat. No. 5,181,080, assigned to theassignee herein and incorporated by reference.

[0068] As described above, the rays in the reflected probe beamcorrespond to different angles of incidence. By monitoring the change inpolarization state of the beams (from the original linear polarization,to elliptical polarization upon reflection), ellipsometric information,such as ψ and Δ, can be determined.

[0069] To determine this information, the beam is first passed through aquarter-wave plate 404 for retarding the phase of one of thepolarization states of the beam by 90 degrees. The beam is then passedthrough a polarizer 408 which functions to cause the two polarizationstates of the probe beam to interfere with each other. The light is thenpassed through an imaging lens 410 an onto a quadrant detector 412. Eachquadrant, 414, 416, 418, and 420 generate separate output signalsproportional to the power of the probe beam striking that quadrant.These signals represents an integration of the intensities of all therays having different angles of incidence with respect to the samplesurface. As described in U.S. Pat. No. 5,181,080, ellipsometricinformation can be obtained if the signals from opposing quadrants 414and 418 are summed and subtracted from the sum of the signals fromopposing quadrants 412 and 420. The output of detector 412 is sent tothe processor 32 for storage and use in characterizing the sample.

[0070] As can be seen, the subject device can be used to provide a largeamount of measurement data in order to better resolve thecharacteristics of the sample. Such complete measurements are oftennecessary in order to determine the composition of a multilayerstructure.

[0071] While the subject invention has been described with reference toa preferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defmed by the appended claims.

1. An apparatus for evaluating the characteristics of a sample,comprising: an intensity modulated pump laser beam, said pump beam beingdirected to a spot on the surface of the sample for periodicallyexciting the sample; means for varying the modulation frequency of saidpump laser beam; a probe laser beam being directed to a spot on thesurface of the sample within a region that has been periodically excitedand is reflected therefrom; a photodetector for measuring the power ofthe reflected probe beam and generating an output signal in responsethereto; steering means for adjusting the lateral separation between thebeam spots on the surface of the sample; and processor for filtering theoutput signal to provide a measure of the magnitude or phase of themodulated optical reflectivity of the sample, said processor furtherfunctioning to control the means for varying the modulation frequency ofthe pump beam and the steering means in a manner so that a plurality ofmeasurements are taken and recorded at a plurality of differentmodulation frequencies and at a plurality of different separationsbetween the beam spots, with the plurality of measurements being used toevaluate the characteristics of the sample.
 2. An apparatus as recitedin claim 1 wherein measurements are taken as the separation between thebeam spots is continuously changed.
 3. An apparatus as recited in claim1 wherein measurements are taken at discrete separation intervalsbetween the beam spots.
 4. An apparatus as recited in claim 1 whereinsaid steering means functions to separate the beams over a range from analigned overlapping position to a spacing of at least 10 microns.
 5. Anapparatus as recited in claim 1 wherein the modulation frequency of thepump laser beam can be at least 100 Mhz.
 6. An apparatus as recited inclaim 1 wherein the modulation frequency of the pump laser beam can bevaried from 100 KHz to 100 MHz.
 7. An apparatus as recited in claim 1wherein the photodetector measures the periodic angular deflections ofthe probe laser beam and wherein the processor uses the angulardeflection measurements to evaluate the characteristics of the sample.8. An apparatus as recited in claim 1 further including a detector meansfor measuring the intensity of rays within the probe beam as a functionof the angle of incidence with respect to the sample surface and whereinthe processor uses the angle of incidence measurements to evaluate thecharacteristics of the sample.
 9. An apparatus as recited in claim 1further including a detector means for measuring the change inpolarization state of the reflected probe beam and wherein the processoruses the polarization state measurements to evaluate the characteristicsof the sample.
 10. An apparatus as recited in claim 9 wherein thedetector means generates an output, signal that represents anintegration of rays within the probe beam having multiple angles ofincidence with respect to the sample surface.
 11. An apparatus asrecited in claim 1 further including a means for varying the size ofeither or both of the pump or probe beam spots and wherein the reflectedpower of the probe beam is measured as the size of the spot is variedand wherein the processor uses the measurements at different spot sizesto evaluate the characteristics of the sample.
 12. An apparatus asrecited in claim 1 further including a broadband polychromatic lightsource for generating a polychromatic probe beam, said polychromaticprobe beam being directed to a spot on the surface of the sample and isreflected therefrom, said apparatus further including a detector meansfor measuring the power of the reflected polychromatic light beam andgenerating a plurality of second output signals corresponding to aplurality of different wavelengths within the polychromatic beam andwherein the processor uses the second output signals to evaluate thecharacteristics of the sample.
 13. An apparatus for evaluating thecharacteristics of a sample comprising: an intensity modulated pumplaser beam, said pump beam being directed to a spot on the surface ofthe sample for periodically exciting the sample; a probe laser beambeing directed to a spot on the surface of the sample within a regionthat has been periodically excited and is reflected therefrom; aphotodetector for measuring the power of the reflected probe beam andgenerating an output signal in response thereto; steering means foradjusting the lateral separation between the beam spots on the surfaceof the sample; and processor for filtering the output signal to providea measure of the magnitude or phase of the modulated opticalreflectivity of the sample, said processor further functioning tocontrol the steering means in a manner so that a plurality ofmeasurements are taken and recorded at a plurality of differentseparations between the beam spots, with the plurality of measurementsbeing used to evaluate the characteristics of the sample.
 14. Anapparatus as recited in claim 13 wherein measurements are taken as theseparation between the beam spots is continuously changed.
 15. Anapparatus as recited in claim 13 wherein measurements are taken atdiscrete separation intervals between the beam spots.
 16. An apparatusas recited in claim 13 wherein said steering means functions to separatethe beams over a range from an aligned overlapping position to a spacingof at least 10 microns.
 17. An apparatus as recited in claim 13 whereinthe modulation frequency of the pump laser beam can be at least 100 MHz.18. An apparatus as recited in claim 13 wherein the modulation frequencyof the pump laser beam can be varied from 100 KHz to 100 MHz.
 19. Anapparatus as recited in claim 13 wherein the photodetector measures theperiodic angular deflections of the probe laser beam and wherein theprocessor uses the angular deflection measurements to evaluate thecharacteristics of the sample.
 20. An apparatus as recited in claim 13further including a detector means for measuring the intensity of rayswithin the probe beam as a function of the angle of incidence withrespect to the sample surface and wherein the processor uses the angleof incidence measurements to evaluate the characteristics of the sample.21. An apparatus as recited in claim 13 further including a detectormeans for measuring the change in polarization state of the reflectedprobe beam and wherein the processor uses the polarization statemeasurements to evaluate the characteristics of the sample.
 22. Anapparatus as recited in claim 21 wherein the detector means generates anoutput signal that represents an integration of rays within the probebeam having multiple angles of incidence with respect to the samplesurface.
 23. An apparatus as recited in claim 13 further including ameans for varying the size of either or both of the pump or probe beamspots and wherein the reflected power of the probe beam is measured asthe size of the spot is varied and wherein the processor uses themeasurements at different spot sizes to evaluate the characteristics ofthe sample.
 24. An apparatus as recited in claim 13 further including ameans for varying the modulation frequency of the pump laser beam andwherein a plurality of measurements are taken at different modulationfrequencies and with different beam spot separations to facilitate theevaluation of the sample.
 25. An apparatus as recited in claim 13further including a broadband polychromatic light source for generatinga polychromatic probe beam, said polychromatic probe beam being directedto a spot on the surface of the sample and is reflected therefrom, saidapparatus further including a detector means for measuring the power ofthe reflected polychromatic light beam and generating a plurality ofsecond output signals corresponding to a plurality of differentwavelengths within the polychromatic beam and wherein the processor usesthe second output signals to evaluate the characteristics of the sample.26. An apparatus for evaluating the characteristics of a samplecomprising: an intensity modulated pump laser beam, said pump beam beingdirected to a spot on the surface of the sample for periodicallyexciting the sample; a probe laser beam being directed to a spot on thesurface of the sample within a region that has been periodically excitedand is reflected therefrom; a photodetector for measuring the power ofthe reflected probe beam and generating an output signal in responsethereto; means for adjusting the spot size of either or both of the pumpor probe beams on the surface of the sample; and processor for filteringthe output signal to provide a measure of the magnitude or phase of themodulated optical reflectivity of the sample, said processor furtherfunctioning to control the spot size adjustment means in a manner sothat a plurality of measurements are taken and recorded at a pluralityof different spot sizes with the plurality of measurements being used toevaluate the characteristics of the sample.
 27. An apparatus as recitedin claim 26 wherein the size of the beam spots is varied from a fewmicrons in diameter to ten microns in diameter.
 28. An apparatus asrecited in claim 26 further including a steering means for adjusting thelateral separation between the pump and probe laser beam spots on thesurface of the sample and wherein a plurality of measurements are takenat different separations between the pump and probe laser beam spots andwherein the processor uses the measurements at spot separations toevaluate the characteristics of the sample.
 29. An apparatus as recitedin claim 26 further including a means for varying the modulationfrequency of the pump laser beam and wherein a plurality of measurementsare taken at different modulation frequencies and with different beamspot separations to facilitate the evaluation of the sample.
 30. Anapparatus as recited in claim 27 wherein the modulation frequency of thepump laser beam can be varied from 100 KHz to 100 MHz.
 31. An apparatusas recited in claim 28 wherein the photodetector measures the periodicangular deflections of the probe laser beam and wherein the processoruses the angular deflection measurements to evaluate the characteristicsof the sample.
 32. An apparatus as recited in claim 28 further includinga detector means for measuring the intensity of rays within the probebeam as a function of the angle of incidence with respect to the samplesurface and wherein the processor uses the angle of incidencemeasurements to evaluate the characteristics of the sample.
 33. Anapparatus as recited in claim 28 further including a detector means formeasuring the change in polarization state of the reflected probe beamand wherein the processor uses the polarization state measurements toevaluate the characteristics of the sample.
 34. An apparatus as recitedin claim 33 wherein the detector means generates an output signal thatrepresents an integration of rays within the probe beam having multipleangles of incidence with respect to the sample surface.
 35. An apparatusas recited in claim 28 further including a broadband polychromatic lightsource for generating a polychromatic probe beam, said polychromaticprobe beam being directed to a spot on the surface of the sample and isreflected therefrom, said apparatus further including a detector meansfor measuring the power of the reflected polychromatic light beam andgenerating a plurality of second output signals corresponding to aplurality of different wavelengths within the polychromatic beam andwherein the processor uses the second output signals to evaluate thecharacteristics of the sample.
 36. An apparatus for evaluating thecharacteristics of a sample, comprising: an intensity modulated pumplaser beam, said pump beam being directed to a spot on the surface ofthe sample for periodically exciting the sample; a probe laser beambeing directed to a spot on the surface of the sample within a regionwhich has been periodically excited and is reflected therefrom; aphotodetector for measuring the power of the reflected probe laser beamand generating a first output signal in response thereto; a broadbandpolychromatic light source for generating a polychromatic probe beam,said polychromatic probe beam being directed to a spot on the surface ofthe sample and is reflected therefrom; detector means for measuring thepower of the reflected polychromatic probe beam and generating aplurality of second output signals corresponding to a plurality ofdifferent wavelengths within the polychromatic probe beam; and processorfor filtering the first output signal to provide a measure of themagnitude or phase of the modulated optical reflectivity of the sample,said processor further functioning to monitor the second output signalswith the first and second output signals being used to evaluate thecharacteristics of the sample.
 37. An apparatus as recited in claim 36wherein the probe beam from the polychromatic light source is directedto the same location as the probe laser beam is directed.
 38. Anapparatus as recited in claim 36 further including a steering means foradjusting the lateral separation between the pump and probe laser beamspots on the surface of the sample and wherein a plurality ofmeasurements are taken at different separations between the pump andprobe laser beam spots.
 39. An apparatus as recited in claim 36 whereinsaid steering means functions to separate the pump and probe laser spotsbeams over a range from an aligned overlapping position to a spacing ofat least 10 microns.
 40. An apparatus as recited in claim 36 furtherincluding a means for varying the modulation frequency of the pump laserbeam and wherein a plurality of measurements are taken at differentmodulation frequencies and with different beam spot separations tofacilitate the evaluation of the sample.
 41. An apparatus as recited inclaim 40 wherein the modulation frequency of the pump laser beam can bevaried from 100 KHz to 100 MHz.
 42. An apparatus as recited in claim 38further including a means for varying the size of either or both of thepump and probe laser beam spots and wherein the reflected power of theprobe laser beam is measured as the size of the spots is varied andwherein the processor uses the measurements at different spot sizes toevaluate the characteristics of the sample.
 43. An apparatus as recitedin claim 38 wherein the photodetector measures the periodic angulardeflections of the probe laser beam and wherein the processor uses theangular deflection measurements to evaluate the characteristics of thesample.
 44. An apparatus as recited in claim 38 further including asecond detector means for measuring the intensity of rays within theprobe laser beam as a function of the angle of incidence with respect tothe sample surface and wherein the processor uses the angle of incidencemeasurements to evaluate the characteristics of the sample.
 45. Anapparatus as recited in claim 38 further including a second detectormeans for measuring the change in polarization state of the reflectedprobe beam and wherein the processor uses the polarization statemeasurements to evaluate the characteristics of the sample.
 46. Anapparatus as recited in claim 45 wherein the second detector meansgenerates an output signal that represents an integration of rays withinthe probe beam having multiple angles of incidence with respect to thesample surface.
 47. A method for evaluating the characteristics of asample comprising the steps of: directing an intensity modulated pumplaser beam to a spot on the surface of the sample for periodicallyexciting the sample; directing a probe laser beam to a spot on thesurface of the sample within a region that has been periodically excitedand is reflected therefrom; varying the modulation frequency of the pumplaser beam; varying the separation between the pump and probe beamspots; measuring the power of the reflected probe beam and generating anoutput signal in response thereto at various modulation frequency andbeam spot separations; and filtering the output signals to provide ameasure of the magnitude or phase of the modulated optical reflectivityof the sample, at various modulation frequencies and beam spotseparations and using said measurements to evaluate the characteristicsof the sample.
 48. A method for evaluating the characteristics of asample comprising the steps of: directing an intensity modulated pumplaser beam to a spot on the surface of the sample for periodicallyexciting the sample; directing a probe laser beam to a spot on thesurface of the sample within a region that has been periodically excitedand is reflected therefrom; varying the separation between the pump andprobe beam spots; measuring the power of the reflected probe beam andgenerating an output signal in response thereto at various beam spotseparations; and filtering the output signals to provide a measure ofthe magnitude or phase of the modulated optical reflectivity of thesample, at various beam spot separations and using said measurements toevaluate the characteristics of the sample.
 49. A method for evaluatingthe characteristics of a sample comprising the steps of: directing anintensity modulated pump laser beam to a spot on the surface of thesample for periodically exciting the sample; directing a probe laserbeam to a spot on the surface of the sample within a region that hasbeen periodically excited and is reflected therefrom; varying the sizeof either of both of the pump and probe beam spots on the sample;measuring the power of the reflected probe beam and generating an outputsignal in response thereto at beam spot sizes; and filtering the outputsignals to provide a measure of the magnitude or phase of the modulatedoptical reflectivity of the sample at various beam spot sizes and usingsaid measurements to evaluate the characteristics of the sample.
 50. Amethod as recited in claim 49 wherein the size of the spot on the sampleis, varied between a few microns in diameter to ten microns in diameter.51. A method for evaluating the characteristics of a sample comprisingthe steps of: directing an intensity modulated pump laser beam to a spoton the surface of the sample for periodically exciting the sample;directing a probe laser beam to a spot on the surface of the samplewithin a region that has been periodically excited and is reflectedtherefrom; measuring the power of the reflected probe beam andgenerating an output signal in response thereto; filtering the outputsignals to provide a measure of the magnitude or phase of the modulatedoptical reflectivity of the sample; directing a broadband, polychromaticlight beam onto a spot on the surface of the sample; measuring theintensity of the reflected polychromatic light beam and generating aplurality of second output signals corresponding to a plurality ofdifferent wavelengths within the polychromatic beam; and evaluating thecharacteristics of the sample using the modulated optical reflectivitymeasurements and the measurements at different wavelengths.
 52. A methodas recited in claim 51 further including the step of varying themodulation frequency of the pump laser and measuring the power of thereflected probe beam at a plurality of modulation frequencies and usingthe measurements to characterize the sample.
 53. A method as recited inclaim 52 further including the step of varying the separation betweenpump and probe laser beam spots on the sample surface and measuring thepower of the reflected probe beam at a plurality of separations andusing the measurements to characterize the sample.
 54. A method asrecited in claim 53 further including the step of varying the size ofeither of both of the pump and probe laser beam spots on the samplesurface and measuring the power of the reflected probe beam at aplurality of pump and probe beam spot sizes and using the measurementsto characterize the sample.
 55. A method as recited in claim 52 whereinmeasurements are taken as the separation between the pump and probelaser beam spots is continuously changed.
 56. A method as recited inclaim 52 wherein measurements are taken at discrete separation intervalsbetween the pump and probe laser beam spots.
 57. A method as recited inclaim 52 wherein the spacing between the pump and probe beam spots isvaried over a range from an aligned overlapping position to a spacing ofat least 10 microns.
 58. A method as recited in claim 52 wherein themodulation frequency of the pump laser beam can be varied from 100 KHzto 100 MHz.
 59. A method as recited in claim 52 further including thestep of measuring the periodic angular deflections of the probe laserbeam and using those measurements to evaluate the characteristics of thesample.
 60. A method as recited in claim 52 further including the stepof measuring the intensity of rays within the probe laser beam as afunction of the angle of incidence with respect to the sample surfaceand using the angle of incidence measurements to evaluate thecharacteristics of the sample.
 61. A method as recited in claim 52further including the step of measuring the change in polarization stateof the reflected probe beam and using the measurements to evaluate thecharacteristics of the sample.
 62. A method as recited in claim 61wherein the step of measuring the change of polarization state of thereflected probe beam includes generating an output signal thatrepresents an integration of rays within the probe beam having multipleangles of incidence with respect to the sample surface.